Make sure remember_dc has its own copy of the model string

When called from the parser the model string is freed right after passing
it to remember_dc. So we need to get our own copy.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Remove autogroup from the preferences and store per file instead

Having two spots to toggle autogroup had always been a clear sign of
insanity. The inconsistent ludicrous semantic of when we remembered the
state of autogroup was even worse.

This finally gets rid of that disaster and drops the autogroup setting
from the preferences and makes it instead a per file property. When you
save a file, it saves the state of the autogroup toggle. This seems much
more useful - you may have files where you want to create trips by
default. And others, where you don't.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>